1. Field of the Invention
This invention relates to a semiconductor element, and more particularly to a semiconductor element equipped with nonvolatile ferroelectric memory.
2. Description of Related Art
Ferroelectric memory (nonvolatile memory) has recently come into use as a type of memory capable of operating at high speeds, low voltages, reduced energy consumption, and the like in response to the need for achieving higher integration in semiconductor memory devices.
Ferroelectric memory is a type of memory that has a ferroelectric memory cell structure based on transistors and ferroelectric capacitors and that utilizes the ability of a ferroelectric film (which is used in capacitors for accumulating and storing electric charges) to reverse and maintain an electric field as a result of a spontaneous polarization occurring in the film itself.
There are, for example, 2T2C (two-transistor, two-capacitor) memory cells in which each ferroelectric memory cell comprises two transistors and two ferroelectric capacitors. 2T2C memory cells have excellent fatigue resistance and can operate stably despite the reduced capacitor polarization brought about by data read/write cycles, but the surface area occupied by each memory cell is considerable, making this type of memory cells unsuitable for higher integration.
In view of this, research has been conducted in order to achieve higher integration by devising 1T1C (one-transistor, one-capacitor) memory cells in which each ferroelectric memory cell comprises a single transistor and a single ferroelectric capacitor, but these cells still have numerous problems that make it impossible to ensure stable operation.
Research has also been performed in recent years into ferroelectric memory mixed LSI circuits in which ferroelectric memory is mounted not only as general-purpose memory but also in system LSI. Such ferroelectric memory mixed LSI circuits were expected to be used in applications that required small chip sizes and low power consumption, such as chips for IC cards.
However, the following typical problems are encountered when, for example, such a ferroelectric memory mixed LSI circuit is created. These problems result from the fact that SBT (SrBi2Ta2O9), PZT (PbZrxTi1−xO3), and other metal oxide films are commonly used as ferroelectric films.
For example, steps for manufacturing a common semiconductor device comprise formation steps such as a step for forming an interlayer insulating film, a step for forming a passivation film, and a molding step, and all these steps are performed in a reductive atmosphere based on hydrogen (H2) gas or the like.
As a result, the characteristics of ferroelectric films (hereinafter abbreviated as “ferroelectric characteristics”) are degraded because these films are reduced by being exposed to a reducing atmosphere.
Specifically, the ferroelectric characteristics undergo considerable degradation because a large amount of hydrogen diffuses into the ferroelectric film by the generation of hydrogen plasma in a step in which, for example, passivation and film forming are performed in a reducing atmosphere.
In this case, a heat treatment can be performed in order to restore the degraded ferroelectric characteristics, but such heat treatment is unacceptable when the wiring material has low heat resistance.
In view of this, methods have been proposed in which the degradation of ferroelectric characteristics is prevented by covering the surface of a ferroelectric capacitor with a coating film to protect the dielectric film. However, the addition of a step for forming a coating film results in higher manufacturing costs, and it is difficult to form a coating film that would have excellent step coverage on the surface of a stepped ferroelectric capacitor.
Consequently, it has conventionally been considered difficult to create a ferroelectric memory mixed LSI circuit by mounting, for example, logic units and other semiconductor devices on the same substrate on which ferroelectric memory is formed.
A need thus existed in the past for a technique in which the aforementioned drawbacks would be overcome in a technologically acceptable manner.